Fix diff patch parser for paths with unsafe chars

This specifically covers the cases where unsafe chars occur in path
names, and git-diff -p will escape those.

From the git-diff-tree manpage:

> 3. TAB, LF, double quote and backslash characters in pathnames are
>    represented as \t, \n, \" and \\, respectively. If there is need
>    for such substitution then the whole pathname is put in double
>    quotes.

This patch checks whether or not this has happened and will unescape
those paths accordingly.

One thing to note here is that, depending on the position in the patch
format, those paths may be prefixed with an a/ or b/.  I've specifically
made sure to never interpret a path that actually starts with a/ or b/
incorrectly.

Example of that subtlety below.  Here, the actual file path is
"b/normal".  On the diff file that gets encoded as "b/b/normal".

     diff --git a/b/normal b/b/normal
     new file mode 100644
     index 0000000000000000000000000000000000000000..eaf5f7510320b6a327fb308379de2f94d8859a54
     --- /dev/null
     +++ b/b/normal
     @@ -0,0 +1 @@
     +dummy content

Here, we prefer the "---" and "+++" lines' values.  Note that these
paths start with a/ or b/.  The only exception is the value "/dev/null",
which is handled as a special case.

Suppose now the file gets moved "b/moved", the output of that diff would
then be this:

     diff --git a/b/normal b/b/moved
     similarity index 100%
     rename from b/normal
     rename to b/moved

We prefer the "rename" lines' values in this case (the "diff" line is
always a last resort).  Take note that those lines are not prefixed with
a/ or b/, but the ones in the "diff" line are (just like the ones in
"---" or "+++" lines).

1 files changed, 75 insertions, 0 deletions

diff --git a/git/test/fixtures/diff_patch_unsafe_paths b/git/test/fixtures/diff_patch_unsafe_paths
new file mode 100644
index 00000000..14375f79
--- /dev/null
+++ b/git/test/fixtures/diff_patch_unsafe_paths
@@ -0,0 +1,75 @@
+diff --git a/path/ starting with a space b/path/ starting with a space
+new file mode 100644
+index 0000000000000000000000000000000000000000..eaf5f7510320b6a327fb308379de2f94d8859a54
+--- /dev/null
++++ b/path/ starting with a space
+@@ -0,0 +1 @@
++dummy content
+diff --git "a/path/\"with-quotes\"" "b/path/\"with-quotes\""
+new file mode 100644
+index 0000000000000000000000000000000000000000..eaf5f7510320b6a327fb308379de2f94d8859a54
+--- /dev/null
++++ "b/path/\"with-quotes\""
+@@ -0,0 +1 @@
++dummy content
+diff --git a/path/'with-single-quotes' b/path/'with-single-quotes'
+new file mode 100644
+index 0000000000000000000000000000000000000000..eaf5f7510320b6a327fb308379de2f94d8859a54
+--- /dev/null
++++ b/path/'with-single-quotes'
+@@ -0,0 +1 @@
++dummy content
+diff --git a/path/ending in a space  b/path/ending in a space 
+new file mode 100644
+index 0000000000000000000000000000000000000000..eaf5f7510320b6a327fb308379de2f94d8859a54
+--- /dev/null
++++ b/path/ending in a space 
+@@ -0,0 +1 @@
++dummy content
+diff --git "a/path/with\ttab" "b/path/with\ttab"
+new file mode 100644
+index 0000000000000000000000000000000000000000..eaf5f7510320b6a327fb308379de2f94d8859a54
+--- /dev/null
++++ "b/path/with\ttab"
+@@ -0,0 +1 @@
++dummy content
+diff --git "a/path/with\nnewline" "b/path/with\nnewline"
+new file mode 100644
+index 0000000000000000000000000000000000000000..eaf5f7510320b6a327fb308379de2f94d8859a54
+--- /dev/null
++++ "b/path/with\nnewline"
+@@ -0,0 +1 @@
++dummy content
+diff --git a/path/with spaces b/path/with spaces
+new file mode 100644
+index 0000000000000000000000000000000000000000..eaf5f7510320b6a327fb308379de2f94d8859a54
+--- /dev/null
++++ b/path/with spaces
+@@ -0,0 +1 @@
++dummy content
+diff --git a/path/with-question-mark? b/path/with-question-mark?
+new file mode 100644
+index 0000000000000000000000000000000000000000..eaf5f7510320b6a327fb308379de2f94d8859a54
+--- /dev/null
++++ b/path/with-question-mark?
+@@ -0,0 +1 @@
++dummy content
+diff --git "a/path/¯\\_(ツ)_|¯" "b/path/¯\\_(ツ)_|¯"
+new file mode 100644
+index 0000000000000000000000000000000000000000..eaf5f7510320b6a327fb308379de2f94d8859a54
+--- /dev/null
++++ "b/path/¯\\_(ツ)_|¯"
+@@ -0,0 +1 @@
++dummy content
+diff --git a/a/with spaces b/b/with some spaces
+similarity index 100%
+rename from a/with spaces
+rename to b/with some spaces
+diff --git a/a/ending in a space  b/b/ending with space 
+similarity index 100%
+rename from a/ending in a space 
+rename to b/ending with space 
+diff --git "a/a/\"with-quotes\"" "b/b/\"with even more quotes\""
+similarity index 100%
+rename from "a/\"with-quotes\""
+rename to "b/\"with even more quotes\""