Add String() methods to parsed types (#48)

This enables clients to move back and forth between parsed objects and
text patches. The generated patches are semantically equal to the parsed
object and should re-parse to the same object, but may not be
byte-for-byte identical to the original input.

In my testing, formatted text patches are usually identical to the
input, but there may be cases where this is not true. Binary patches
always differ. This is because Go's 'compress/flate' package ends
streams with an empty block instead of adding the end-of-stream flag to
the last non-empty block, like Git's C implementation. Since the streams
will always be different for this reason, I chose to also enable default
compression (the test patches I generated with Git used no compression.)

The main tests for this feature involve parsing, formatting, and then
re-parsing a patch to make sure we get equal objects.

Formatting is handled by a new internal formatter type, which allows
writing all data to the same stream. This isn't exposed publicly right
now, but will be useful if there's a need for more flexible formatting
functions in the future, like formatting to a user-provided io.Writer.

gitdiff/testdata/string/binary_modify.patch

@@ -0,0 +1,9 @@
+diff --git a/file.bin b/file.bin
+index a7f4d5d6975ec021016c02b6d58345ebf434f38c..bdc9a70f055892146612dcdb413f0e339faaa0df 100644
+GIT binary patch
+delta 66
+QcmeZhVVvM$!$1K50C&Ox;s5{u
+
+delta 5
+McmZo+^qAlQ00i9urT_o{
+

gitdiff/testdata/string/binary_new.patch

@@ -0,0 +1,11 @@
+diff --git a/file.bin b/file.bin
+new file mode 100644
+index 0000000000000000000000000000000000000000..a7f4d5d6975ec021016c02b6d58345ebf434f38c
+GIT binary patch
+literal 72
+zcmV-O0Jr~td-`u6JcK&{KDK=<a#;v1^LR5&K)zQ0=Goz82(?nJ6_nD`f#8O9p}}{P
+eiXim+rDI+BDadMQmMsO5Sw@;DbrCA+PamP;Ng_@F
+
+literal 0
+HcmV?d00001
+

gitdiff/testdata/string/copy.patch

@@ -0,0 +1,4 @@
+diff --git a/file.txt b/numbers.txt
+similarity index 100%
+copy from file.txt
+copy to numbers.txt

gitdiff/testdata/string/copy_modify.patch

@@ -0,0 +1,21 @@
+diff --git a/file.txt b/numbers.txt
+similarity index 57%
+copy from file.txt
+copy to numbers.txt
+index c9e9e05..6c4a3e0 100644
+--- a/file.txt
++++ b/numbers.txt
+@@ -1,6 +1,6 @@
+ one
+ two
+-three
++three three three
+ four
+ five
+ six
+@@ -8,3 +8,5 @@ seven
+ eight
+ nine
+ ten
++eleven
++twelve

gitdiff/testdata/string/delete.patch

@@ -0,0 +1,16 @@
+diff --git a/file.txt b/file.txt
+deleted file mode 100644
+index c9e9e05..0000000
+--- a/file.txt
++++ /dev/null
+@@ -1,10 +0,0 @@
+-one
+-two
+-three
+-four
+-five
+-six
+-seven
+-eight
+-nine
+-ten

gitdiff/testdata/string/mode.patch

@@ -0,0 +1,3 @@
+diff --git a/file.txt b/file.txt
+old mode 100644
+new mode 100755

gitdiff/testdata/string/mode_modify.patch

@@ -0,0 +1,10 @@
+diff --git a/script.sh b/script.sh
+old mode 100644
+new mode 100755
+index 7a870bd..68d501e
+--- a/script.sh
++++ b/script.sh
+@@ -1,2 +1,2 @@
+ #!/bin/bash
+-echo "Hello World"
++echo "Hello, World!"

gitdiff/testdata/string/modify.patch

@@ -0,0 +1,16 @@
+diff --git a/file.txt b/file.txt
+index c9e9e05..7d5fdc6 100644
+--- a/file.txt
++++ b/file.txt
+@@ -3,8 +3,10 @@ two
+ three
+ four
+ five
+-six
++six six six six six six
+ seven
+ eight
+ nine
+ ten
++eleven
++twelve

gitdiff/testdata/string/new.patch

@@ -0,0 +1,16 @@
+diff --git a/file.txt b/file.txt
+new file mode 100644
+index 0000000..c9e9e05
+--- /dev/null
++++ b/file.txt
+@@ -0,0 +1,10 @@
++one
++two
++three
++four
++five
++six
++seven
++eight
++nine
++ten

gitdiff/testdata/string/new_empty.patch

@@ -0,0 +1,3 @@
+diff --git a/file.txt b/file.txt
+new file mode 100644
+index 0000000..e69de29

gitdiff/testdata/string/new_mode.patch

@@ -0,0 +1,16 @@
+diff --git a/file.sh b/file.sh
+new file mode 100755
+index 0000000..c9e9e05
+--- /dev/null
++++ b/file.sh
+@@ -0,0 +1,10 @@
++one
++two
++three
++four
++five
++six
++seven
++eight
++nine
++ten

gitdiff/testdata/string/rename.patch

@@ -0,0 +1,4 @@
+diff --git a/file.txt b/numbers.txt
+similarity index 100%
+rename from file.txt
+rename to numbers.txt

gitdiff/testdata/string/rename_modify.patch

@@ -0,0 +1,18 @@
+diff --git a/file.txt b/numbers.txt
+similarity index 77%
+rename from file.txt
+rename to numbers.txt
+index c9e9e05..a6b31d6 100644
+--- a/file.txt
++++ b/numbers.txt
+@@ -3,8 +3,9 @@ two
+ three
+ four
+ five
+-six
++  six
+ seven
+ eight
+ nine
+ ten
++eleven